Like true love, the course of evolution does not run smooth. Species evolve in fits and starts, and some aspects of their biology change more quickly than others. In
The evolutionary ways of gorillas, chimps and humans probably parted about 6 million years ago -- before this they were a single, ancestral ape species. We can measure how much change there has been since that split by comparing the DNA sequences of today's species. Wu's group looked at three genes that make proteins involved in the manufacture and function of sperm. These are known to have changed rapidly since the different species of great ape diverged.
They wanted to find out if these changes had been favoured by natural selection, or whether they were the result of selection becoming weaker and allowing mutations to accumulate in the genome more or less at random, without evolution weeding them out.
This latter possibility is known as neutral evolution, and reflects the idea that much -perhaps most -- of the change in the genome through time has no effect on an organism's chances of survival. One reason why this might be is that many different DNA sequences can result in the same sequence of protein building blocks, called 'amino acids'. For example, the DNA sequences 'CGU' and 'CGC' both mean the amino acid 'arginine' to the cell's protein-assembly machinery. So if the last letter (technically known as a 'base') mutates from 'U' to 'C' the protein won't change -- these are called synonymous mutations.
Wu's team compared the ratio of synonymous to non-synonymous mutations in the sperm genes. A high proportion of synonymous mutations would show that change in the genome was probably random, and not the result of natural selection. In fact they found that a large majority of changes were non-synonymous -- that is, they were alterations that changed the protein made from a gene -- and had therefore been favoured by evolution.
So what selective force caused these genes to change so quickly? Like so much in life, they seem to be driven by sex. Broadly speaking, female chimpanzees mate with many males over a short period of time, female gorillas with only one, a dominant silverback, and the mating patterns of female humans fall somewhere between the two. This means that in chimps and humans, sperm are more likely to find themselves mixing with the sperm of other males inside a female. Only one of them can fertilize the egg, so competition is fierce. This is why chimpanzees have much larger testicles, relative to the size of their bodies, than gorillas; humans, again, are intermediate.
So one would expect natural selection to be a stronger influence on sperm in chimps and humans. And, indeed, sperm protein genes have changed much more as modern humans and chimpanzees evolved than they have in the course of gorilla evolution.
It seems a good bet, then, that sexual competition between males has driven some rapid genetic change in humans and chimpanzees and possibly in many other species, as genes involved in male reproduction are often evolutionary speed merchants. What's more this new research is another tentative link between our knowledge of behaviour and ecology and the changes we see at the level of DNA.